The present invention relates to industrial process monitoring and control systems. More specifically, the present invention relates to a field device, such as a process transmitter or process controller, having a normal operating mode and a diagnostic self-test mode in which condition of a piezoelectric transducer is tested.
A process transmitter includes a sensing element or transducer that responds to a process variable, and signal conditioning and processing circuitry to convert the sensed variable into a transmitter output that is a function of the sensed process variable. The term “process variable” refers to a physical or chemical state of matter or conversion of energy. Examples of process variables include pressure, temperature, flow, conductivity, PH, and other properties. Process transmitters are typically used to monitor process variables and send measurement values back to a control room in a chemical, petroleum, gas, pharmaceutical, or other fluid processing plant. Often these environments are subject to harsh and varying environmental conditions.
A process controller includes an actuator or transducer and actuator drive circuitry that convert an electrical command to a physical output. The process controller, for example, may open or close a valve, or change a process parameter such as pressure in response to a command received from a control room in a fluid processing plant.
All electrical components, including the transducers and circuitry of field devices, have the potential to malfunction or fail. Such a malfunction could result in a faulty measurement being sent by the process transmitter to the control room, or the failure of a process controller perform a commanded action. Routine testing by a skilled technician can detect problems with a field device, but that requires the technician to physically access the field device. In facilities having a substantial number of distributed field devices, there is a practical limit to how frequently a technician can visit and test each field device.
Piezoelectric transducers are used as sensors in some process transmitters. The piezoelectric sensors may be either passive sensors which produce an electrical sensor output as a function of applied force, or active sensors that transmit and receive signals. Piezoelectric transducers also are used as actuators or drive elements in some process controllers.
One example of a process transmitter that uses a piezoelectric transducer detects gas/steam leakages in a hazardous environment. The transmitter detects whether safety valves are open or closed, and blow through or plugged conditions within steam traps. The transmitter includes a piezoelectric sensor, signal conditioning circuits, and a microcontroller. Measurement values may be sent by the transmitter to a control room over a wired loop, a wired transmission bus, or by wireless transmission such as through a wireless mesh network. The piezoelectric sensor is excited by vibrations in the ultrasonic frequency range, for example, that are produced when a steam trap fails.
Because a process transmitter used to detect gas/steam leakages is typically deployed in hazardous/safety critical applications, the process transmitter would desirably have self-testing or self-diagnosing capability. The electronics can be verified using a diagnosing routine that is executed often. On the other hand, self-testing by the processor of the function of the piezoelectric transducer has not been available.
One way to check whether the piezoelectric transducer of a process transmitter used to detect gas/steam leakages is to deliberately produce a leak to verify piezoelectric sensor functionality. This technique, however, has significant disadvantages. First, in the case of safety valves, a penalty is levied for the gas that is released into the atmosphere during the diagnostic procedure. Second, producing a deliberate leak to verify transducer functionality does not work in the case of steam traps.